Field of the Invention
The present invention relates to a focusing device, a control method therefor, a storage medium storing a control program therefor, and an image pickup apparatus, and in particular, relates to a technique about autofocus (AF) used in an image pickup apparatus.
Description of the Related Art
In general, an image pickup apparatus, such as a digital camera and a video camera, uses what is called contrast AF that detects an in-focus position on the basis of contrast of an image pickup signal obtained from an image pickup device, such as a CCD and a MOS sensor. In the contrast AF, a contrast evaluation value that shows a degree of contrast is computed on the basis of an image pickup signal obtained as a result of photographing while moving a focusing lens along an optical axis. Then, the focusing lens is moved to the in-focus position at which the contrast is maximized along the optical axis on the basis of the contrast evaluation value concerned.
However, since the contrast AF needs to obtain the contrast evaluation value for every image pickup cycle, it is necessary to lengthen the image pickup cycle when Illuminance of an object is low. As a result, focusing time required until detecting the in-focus position increases when the illuminance of the object is low.
On the other hand, there is a method for shortening the focusing time in a low illuminance environment. For example, Japanese Laid-Open Patent Publication (Kokai) No. 2010-243790 (JP 2010-243790A) discloses a method that shortens the image pickup cycle as with the case of high illuminance when an object has high contrast even if the illuminance in a photographing environment is low.
However, according to the method disclosed in the above-mentioned publication, when the contrast AF is started for an object (a human face, for example) of which contrast varies according to a defocus amount in a low illuminance photographing environment and in a defocus state, it is determined that both of the illuminance and the contrast of the object are low. As a result, a long image pick-up cycle is set up and the focusing time required until detecting the in-focus position becomes long.